A reoccurring problem in medicine is that, due to the lack of specificity of the agents used for treatment of illnesses, the patient is often the recipient of a new set of maladies from the therapy. This scenario is common especially in the treatment of the various forms of cancer.
An approach taken to circumvent the nonspecificity of the agents used to treat diseases is to couple an agent to a carrier that possesses some degree of specificity. A number of molecules have been utilized as carriers in agent delivery systems, but with limited success. Carrier molecules such as liposomes, proteins, and polyclonal antibodies have been used in conjunction with a broad spectrum of pharmaceutical or cytotoxic agents including radioactive compounds, agents which bind DNA, antimetabolites, agents which act on cell surfaces, and protein synthesis inhibitors.
With the discovery of a method to isolate antibodies with a single specificity, i.e., monoclonal antibodies (MAbs), came the hope that agents could now be delivered to selected cells via "immunoconjugates." "Immunoconjugates" are covalently bonded hybrid molecules composed of a recognition portion, such as an antibody molecule, an antibody fragment, or a functional equivalent thereof, and a biologically active portion, such as a toxin, toxin fragment, a drug, a biological response modifier, or a radioisotope. Immunoconjugates have enormous potential as potent anti-tumor agents, due to the selectivity imparted to the hybrid molecules by the antibody portion of the immunoconjugate. The exquisite selectivity of antibodies or antibody fragments permits delivery of increased dose of cytotoxic, inhibitory or radiolabled moieties to a defined population of cells.
Although the MAb carrier systems have gone far to solve the cell-specificity problem, other problems remain. In particular, the design of the compound used to link the agent to the MAb is important. First, where the agent is only active, or at least more potent, when free from the MAb carrier, the linker needs to be cleavable in order to release the agent. Second, where the agent is only active, or at least more potent, when none of the linker remains attached following the cleavage, the labile bond must be the one formed between the linker and the agent. Third, the type of labile bond used should be chosen on the basis of the location, i.e., inside or outside a cell, of the release-inducing factor.
A number of different cleavable linker groups have been described previously. The mechanisms for release of an agent from these linker groups include cleavage by reduction of a disulfide bond, by irradiation of a photolabile bond, by hydrolysis of derivatized amino acid side chain, by serum complement-mediated hydrolysis, and acid-catalyzed hydrolysis. Some of these mechanisms are susceptible to release of the agent prior to having reached the specific cell, tissue or organ. Other of these mechanisms will provide faithful external delivery, however, they are inappropriate where the actual target site of the agent is inside a cell. Where an agent activates or inactivates a cell by binding to an intracellular component, the carrier-agent conjugate must be internalized and then the agent released.
A way to achieve internalization of a carrier-agent conjugate is to take advantage of a cell's receptor-mediated endocytosis pathway. Antibodies are one example of a carrier that will bind to cell surface receptors and be internalized. Receptors which are internalized by receptor-mediated endocytosis pass through acidified compartments known as endosomes or receptosomes. Thus, the carrier-agent conjugate will be exposed transiently to an acidic pH.
Blattler et al., in U.S. Pat. No. 4,569,789, describe a drug delivery system which is formed by reaction of an active substance with a maleic anhydride moiety. The active substance is released upon cleavage of the amide bond. The patent purports that cleavage occurs under mildly acidic conditions, yet the patent discloses that at pH 5 only about 15% is cleaved after five hours. Even at pH 4 for five hours, less than 50% is cleaved.
Thus, there is a need in the art for a carrier-agent conjugate which releases the agent by cleavage under mild conditions. The present invention fulfills this need and further provides other related advantages.